(Invited) Electrodeposition in Microporous Silicon from the Viewpoint of Hydration Property: Effect of Coexisting Ions in Zinc Electrodeposition

A chemical reaction in a nanospace is decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. We illustrate how to overcome this problem for platinum electrodeposition within nanoporous silicon electrodes. The surface-induced hydration structure of reactants is essential. We make nanopore surfaces hydrophobic by covering them with organic molecules and adopt platinum complex ions with sufficiently large sizes as reactants. Such ions, which are only weakly hydrated, are excluded from the bulk aqueous electrolyte solution to the surface, and thus they are hydrophobic in this sense. When the ion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. Using statistical-mechanical theory for confined molecular liquids, we show that this change originates from a surface-induced phase transition. When the affinity of the surface with water was gradually reduced with fixing the ion concentration, qualitatively the same transition phenomenon was observed. We examine how the platinum electrodeposition behavior is affected by the cation species coexisting with the anions. We compare the experimental results obtained using three different cation species: K⁺, (CH₃)₄N⁺, and (C₂H₅)₄N⁺. It is shown that the threshold concentration, beyond which the electrochemical deposition within nanopores is drastically accelerated, is considerably dependent on the cation species. The threshold concentration becomes lower as the cation size increases. Finally, we extend the concept to electrodeposition of other metals such as zinc. Zinc is normally deposited as dendritic crystals under a high current density. However, the dendritic deposition is suppressed by using microporous silicon electrodes. In addition, by choosing the coexisting ions, the dendritic growth of zinc is strongly suppressed. This strategy is crucial for the development of next-generation rechargeable batteries using a metal anode.